It has been known that a metal nanoparticle having a nanometer scale size has characters different from the metal in a bulky size. A metal nanoparticle has characters, for example, of melting point depression, quantum effect, specific surface area increase, and the like. It has been found recently that a new character is added by aggregating metal nanoparticles, and the related studies have been energetically carried out. For example, a large number of reports have appeared on applications of a metal nanoparticle aggregate, such as a photocatalytic reaction, a Raman enhancing element, a chemical formula sensor, and the like.
For example, in Non Patent Literature 1 is described that a 3-dimensional aggregate of zinc oxide fine particles enhanced light scattering and increased the photoelectric conversion efficiency in a solar cell. Further, in Non Patent Literature 2 is described that a 3-dimensional superlattice structure of gold nano-particles remarkably intensified a Raman scattering spectrum. Further, in Patent Literature 1 is described that by arraying regularly colloidal particles infrared light was reflected efficiently and the transmittance of visible light was increased. In Non Patent Literature 3 is described that colloidal fine particles encapsulating a drug were used as a drug delivery system.
While studies on a metal nanoparticle aggregate are progressing, several methods for aggregating metal nanoparticles have been found and reported.
For example Non Patent Literature 4 discloses a method for aggregating metal nanoparticles on a substrate provided with a polymer. Further, Non Patent Literature 5 discloses a method for aggregating metal nanoparticles using a template composed of silica particles.